io INTRODUCTION 



Acanthophis, or in a series of horny segments which 

 are vibrated like a rattle, as in the well-known 

 Crotalus of America, to which we shall refer again at 

 the end of this chapter. In some of the burrowing 

 Uropeltidae, the very short tail is obliquely truncated, 

 with indurated shields above, and acts as a trowel. 

 And, finally, the marine snakes of the subfamily 

 Hydrophiinae are distinguished by a strongly com- 

 pressed, oar - shaped tail, with rounded vertical 

 outline. In a few forms, arboreal or aquatic, the 

 tail is more or less prehensile. 



Males generally have a longer tail than females, 

 and the genital organs, which are lodged in its base, 

 cause a swelling of that region which contrasts with 

 the more gradually tapering extremity of the female, 

 thus affording a means of distinguishing the sexes 

 externally in the majority of snakes. 



The rudimentary hind limbs of Boid snakes, to be 

 mentioned further on in the description of the 

 skeleton, terminate in a claw-like horny spur, which 

 appears on each side of the vent in the male, and 

 sometimes also, though less distinctly, in the 

 female. These spurs are probably of use in facilita- 

 ting the pairing, an explanation which appears the 

 more plausible from the fact that the snakes pro- 

 vided with them have the copulatory intromittent 

 organs destitute of the erectile spines which are 

 present in most others. 



The head varies in shape as much as the body. 



